Impedance,
or "Z" is the name for the resistance of flow of alternating current.
It is a force that opposes the flow of current at a specific frequency.
This opposition force is measured in ohms and is written using the Greek
letter "W".

High Impedance

A high impedance
microphone has an impedance from 10,000 to 100,000 ohms. The most
common values range between 10,000 and 50,000 ohms. About the only
advantage of high impedance microphones is lower cost. However there
are many disadvantages. The cable length is critical. A 10,000 ohm
microphone is already experiencing loss at 10,000 Hertz with only 6' of
cable! The maximum cable length should be around 10'. A 50,000 ohm
microphone will have lost half of its high frequencies with only 10' of
cable. As a result, the sound is muffled.

High impedance
microphones also use unbalanced cable. This means that the very small
signal is being carried in one inner conductor and the outside shield.
As the cable length increases, the unbalanced cable will have an
increasing chance to pick up stray signals from dimmers, fluorescent
lights and radio interference (RFI).

As can be seen, there is
really no application for high impedance microphones in high quality
sound systems.

Low Impedance

Low impedance
microphones have an impedance between 50 and 600 ohms, with the most
common range being 150 to 250 ohms. A major advantage of low impedance
mics is their ability to drive very long cables.

Low impedance
microphones usually use a balanced output. This means there are two
inner conductors to carry the microphone signal with an outer shield.
the outer shield provides an electrical shield for the microphone
signal, which results in better immunity to noise and interference over
single conductor cable. Due to these factors, a balanced, low impedance
microphone cable can go great lengths without degradation. A cable like
Belden 8412 can be run 437' with a 150 ohm microphone with a very slight
loss (1 dB) at 10,000 Hertz.

Low impedance, balanced
lines are the best option when high quality sound is important.